Rates of women who are opting for preventive mastectomies, such as Angeline Jolie, have increased by an estimated 50 percent in recent years, experts say. But many doctors are puzzled because the operation doesn't carry a 100 percent guarantee, it's major surgery -- and women have other options, from a once-a-day pill to careful monitoring.

“They’d hack body fluids all over me and I’d never get sick. Sometimes I’d actually wish for a sick day,” the 63-year-old nurse from San Mateo, Calif., says with a smile.

Pagendarm doesn’t know how she got such a souped-up immune system. She just knows that most of her friends are envious.

Now scientists may have figured out why people like Pagendarm almost never get sick.

In an intriguing new study, researchers isolated a biological marker that appears to predict who is most likely to catch cold. The hope is that the findings will help scientists discover lifestyle changes that will protect people’s immune systems and help them function more like Pagendarm’s.

“The provocative finding is that this is a very stable marker of disease susceptibility that begins to emerge in early adulthood,” says the study’s lead author, Sheldon Cohen, a professor of psychology at Carnegie Mellon University in Pittsburgh.

The new study, published in the Journal of the American Medical Association, followed 152 healthy volunteers aged 18 to 55 who were purposely exposed to the common cold virus via nose drops containing rhinovirus, a sacrifice for which they were paid $1,000 each. Then they were quarantined for five days.

Before they were exposed to the virus, the volunteers were asked to give a blood sample so Cohen and his colleagues could take a closer look at a type of white blood cell that is known to fight off viruses. Like all cells in the human body, these immune cells have structures called telomeres capping the chromosomes inside them.

Telomeres are very much like the little plastic tip of a shoelace in that they keep everything from unraveling. Telomeres get shorter every time a cell divides, and they tend to get shorter as a person ages.

Shortened telomeres, however, don’t protect what lies beneath them. So, white blood cells with short telomeres don’t work as well and have trouble making copies of themselves to fight off infections, the scientists explained.

So they tracked the length of the telomeres in the patients exposed to the cold virus, with significant results.

Five days after they were exposed to the virus, 69 percent of the volunteers had developed respiratory infections, meaning that researchers could easily find virus particles in the volunteers’ nostrils. Twenty-two percent had developed actual symptoms of a cold, including congestion and increased nasal discharge.

Among those with the shortest telomeres, the rate of infection was 77 percent compared with 50 percent among those with the longest telomeres.

The really telling stat was how many were actually suffering from cold symptoms. Among volunteers with the shortest telomeres, 26 percent showed measurable signs of a cold, compared with 13 percent among those with the longest telomeres.

What's more, researchers found that telomere length became an even more important marker of whether people would get sick as the volunteers aged. Beginning at about age 22, telomore length started to predict whether they'd develop infections.

Earlier research has found that telomere length appears to be influenced by lifestyle factors, including stress. Past studies have found short telomeres in adults who had experienced psychological trauma, for instance.

“It’s a small study, but one that opens the door for us to look further,” says Rita Effros, an expert on telomeres who is unaffiliated with the new research. She’s a professor of pathology and laboratory medicine at the David Geffen School of Medicine at the University of California, Los Angeles.

“But it’s a very exciting study because it suggests that there might be something we can do when we are young in terms of lifestyle changes or stress reduction,” Effros adds.